nvme: move nvme_setup_flush and nvme_setup_rw to common code

And mark them inline so that we don't slow down the I/O submission path by
having to turn it into a forced out of line call.

Signed-off-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: Keith Busch <keith.busch@intel.com>
Signed-off-by: Jens Axboe <axboe@fb.com>